There has been a definite shift by electronics manufacturers toward fabricating products by mounting individual, unpackaged, integrated circuit chips directly to a substrate, e.g., a circuit board. This type of fabrication, referred to as "chip-on-board" assembly, allows for reduced costs since the expense of packaging each chip in a plastic or ceramic package prior to attachment is avoided. Further, chip-on-board assembly allows for a greater density of circuits on a given size circuit board, which is also advantageous.
Currently, individual semiconductor chips are electrically connected to a substrate by first applying a quantity of solder (e.g., a solder "bump") to each of a plurality of contacts on the upper surface of the chip. The chip is then inverted and placed on the substrate so that each solder bump is in aligned contact with a corresponding metallized area on the substrate. Thereafter, the solder bumps are reflowed to achieve a solder bond between the chip and the substrate. Attachment of a chip to the substrate in this manner is known as "flip-chip" bonding. As an alternative to flip-chip bonding, each bond site on the chip may be wire-bonded to a corresponding metallized area on the substrate. Chips may also be connected by using an intermediate tape circuit bonded to both the chip and to the circuit board. This type of bonding is known as "Tape Automated Bonding" (TAB).
Flip-chip, TAB, and wire bonding incur the disadvantage that repairs to the substrate following component attachment cannot be made easily. Removal of a defective chip previously bonded to the substrate often results in damage to the metallized areas on the substrate to which the chip was previously bonded. Repair and/or refurbishing of the metallized areas is often difficult, if not impossible, due to their fragile nature and their close spacing (i.e., fine pitch). As a consequence, chip replacement is generally not practical, resulting in significant scrap costs.
Attempts have been made to solve the repair problem by connecting each chip to the substrate via a slab of anisotropically conductive material sandwiched between the chip and the substrate. The anisotropically conductive material provides electrically conductive paths in the z direction between individual contacts on the chip and corresponding metallized areas on the substrate while providing sufficient impedance in the lateral direction between adjacent paths. Heretofore, the disadvantage associated with the use of anisotropically conductive polymer materials is that some type of fixture has been required to secure the chip and the anisotropically conductive material to the substrate, which adds to the cost and complexity of the fabrication process. It is possible to alleviate the need for a fixture by using an adhesive-type anisotropically conductive material. However, the use of an adhesive-type, anisotropically conductive material would likely give rise to an adhesive residue on the metallized areas on the substrate following removal of the material to replace a defective chip, the same type of problem incurred by conventional chip attachment techniques.
Thus, there is a need for an inexpensive and simple technique for interconnecting an integrated circuit chip to a substrate, while facilitating replacement of a defective chip without damage to the substrate.